An image-pickup device converting light into electrical signals provided for, for example, digital cameras such as digital still cameras, digital video cameras, and digital cameras that can record both still and moving images, includes microscopic photodiodes disposed in rows and columns on the surface of the device chip. The photodiodes are semiconductors generating electrical charges in response to the strength of radiated light. Since the photodiodes themselves cannot distinguish colors, color components of the radiated light needs to be received to capture color images.
A single-chip color image-pickup device provided for common digital cameras or video cameras includes three-color or four-color filters disposed in a mosaic pattern on the photodiodes, so that each of the photodiodes responds to the respective color component. The cameras that utilize RGB color filters disposed in a mosaic pattern, in other words, which capture images with a single-chip image-pickup device, are referred to as single-chip cameras in contrast to, for example, three-chip cameras that utilize three separate image-pickup devices for RGB (red, green, and blue, i.e. the three primary colors of light).
A CCD (charge coupled device) and a CMOS (complementary metal-oxide semiconductor) sensor, for example, are the prevailing sensors employed as the image-pickup device.
CCD originally indicated overall devices having a function of transmitting electrical charges in a bucket-brigade fashion, but nowadays, CCD in general indicates a CCD image-pickup device utilizing a CCD for reading out the electrical charges. The CCD is widely employed in imaging devices such as digital cameras and image scanners that capture images.
The CCD image-pickup device is composed of photodiodes and circuits successively extracting and transmitting electrical charges generated by the photodiodes. Specifically, the CCD image-pickup device includes the photodiodes and electrodes disposed in a grid pattern on a silicon substrate (the number of the grid sections corresponds to the number of pixels of the CCD). As in the case of film, when optical images are focused at the sections of the grid through lenses, each of the photodiodes generates electrical charges in response to the strength of received light. By alternately applying voltages to the electrodes, the accumulated electrical charges move to the adjacent electrodes as if the electrical charges are pushed by a pump, and are finally output one by one. Amplifying this output and converting it into digital values generates two-dimensional image data. For moving images, this process is endlessly repeated to generate moving images as successive still images.
The CMOS sensor is an image-pickup device provided with a CMOS switch for reading out the electrical charges instead of a CCD, and each of the pixels is composed of a photodiode and a switch utilizing a CMOS transistor. That is to say, the photodiodes disposed in a grid pattern are individually provided with a switch, and the electrical charges are read out at each pixel by changing over the switches in turn. This structure can provide various advantages. For example, smears, which are peculiar to the CCD, do not occur in theory, power consumption is low (approximately one-tenth of the CCD), and a single low-voltage can drive the CMOS sensor. Furthermore, since production lines for general chips can be utilized to produce the CMOS sensor together with peripheral functions as one chip, the CMOS sensor can be made smaller with lower costs. However, measures for noise caused by, for example, switching are required for practical application of the CMOS sensor as an image-pickup device.
The number of sensors on a solid-state image-pickup device, i.e. the number of pixels of captured images, for digital video cameras is one million pixels or less in most cases. Meanwhile, for digital still cameras, the number of pixels is increasing from one million or two million, to three million due to improvements in recent semiconductor technology and higher resolution.
Moreover, digital cameras that can record both still and moving images have been developed and come into widespread use in recent years.
However, the time required for reading out all the pixels becomes longer as the number of pixels of the captured images increases. Unlike the still images, when the readout time for moving images becomes long, smooth moving images cannot be produced.
To solve this problem, readout methods in which only some of the pixels are extracted are available when high resolution is not required.
So far, especially when a CCD is employed as an image-pickup device, some extracting methods are employed to display moving images and the like. For example, for 2×2 pixels (four pixels of Gr, R, B, and Gb), in the method shown in FIG. 1, only two pixels are extracted from eight pixels and the rest of the pixels are decimated, or, in the method shown in FIG. 2, only two pixels are extracted from twelve pixels and the rest of the pixels are decimated.
However, since the methods shown in FIGS. 1 and 2 cannot extract the pixels at even intervals, and moreover, do not utilize all the pixels, aliasing occurs in the displayed or recorded moving images.